Ventilated primary bar for current transformer



Dec. 8, 1964 H. A. THORPE 3,160,843

VENTILATED PRIMARY BAR FOR CURRENT TRANSFORMER Filed Aug. 3, 1961 fizlmdli Hwy (19. 2720;

United States Patent ()fiice 3,160,843 Patented Dec. 8, 1964 3,160,843VENTILATEE) PRIMARY BAR FGR CURRENT TRANSFGRMER Harry A. Thorpe,Somersworth, N.H., assignor to General Electric Company, a corporationof New York lied Aug. 3, 1961, Ser. No. 129,092

2 Claims. (Cl. 336-174) This invention relates to current transformers,and more particularly to a ventilated primary for use with a currenttransformer.

As is well known to those familiar with the general field of currenttransformers, it is normal to use a singleturn primary with such currenttransformers. The single turn primary is usually of a solid piece ofcopper material, either a cylindrical rod or a fiat strip or bar,extending through an opening in the secondary winding. The secondarywinding is generally of a toroidal configuration and is embedded in acasing of electrical insulating material. An example of these types ofcurrent transformers are found in Patent No. 2,436,636, which shows acylindrical rod primary, and Patent No. 2,618,689, which shows a fiatbar primary. Where a cylindrical rod is used, it is necessary either tosecure flat terminal pads to the end of such bar for connecting suchprimary to the bus bars or conductors of the electrical system on whichit is to be used. Such fiat terminal pads connected to the end of acylindrical rod is shown in the aforementioned Patent 2,436,636. It hasalso been known to fiatten the ends of the cylindrical rod to form thedesired primary pads. An example of this latter type is shown in Patent2,815,493. Of course, where a flat bar is used for the primary, the endsof such bar form the desired terminal pads without the necessity ofdoing anything more than drilling the necessary holes for the desiredconnection.

As will be understood, when the primary of a current transformer issubjected to high currents, the primary begins to heat up. The portionof the primary which is within the current transformer tends to heat .ata much faster rate, and to a much higher degree than the portions of theprimary which are exterior of the current transformer. This heating ofthe primary limits the amount of current which can be carried by anygiven primary, and thus limits the rating of the current transformerusing such primary. Furthenwhere it is necessary to weld terminal padstothe primary, extreme care must be taken to prevent damaging theinsulation of the current transformer, which surrounds such primary.

From the above, it will be apparent that in the current transformerfield there is a present need for a primary which will enable largercurrents to be utilized with a given size current transformer. Further,it is clearly desirable to provide a primary to which terminal pads maybe attached, without any danger of damaging the insulation of thecurrent transformer, which surrounds such primary. Of course, it isalways desirable to produce a current transformer which will be moreefiicient and less expensive to manufacture than those presentlyavailable.

Therefore, it is an object of this invention to provide a currenttransformer having a primary with ventilation to lessen the heat builtup in such primary.

It is a further object of this invention to provide a currenttransformer with a primary which uses less material than equivalentrated primaries of present day current transformers.

It is a futher object of this invention to provide a cur-.

rent transformer with a primary whereby the terminal pads may be weldedthereto, without damaging the insulation about such primary.

A still further object of this invention is to provide an improvedcurrent transformer, having a novel primary, which is less expensive tomanufacture than present day current transformers.

Briefly, this invention in one form comprises a current transformerhaving a toroidal-shaped secondary enclosed in an electrically insulatedcasing. The casing is provided with a window opening therein, and aprimary in the form of a hollow cylindrical tube fits within such windowopening. Terminal pads are provided at each end of the primary tube,being welded thereto to opposite sides of said tube across the diameterthereof.

The invention which it is desired to protect will be specifically setforth and distinctly claimed in the claims appended hereto. However, itis believed that this invention, and the manner in which its objects andadvantages are obtained, as well as other objects and advantagesthereof, will be better understood from the following detaileddescription of a preferred embodiment thereof when taken in connectionwith the accompanying drawing, wherein:

FIGURE 1 is a perspective view of a current transformer showing one formof improved primary of this invention used therewith; and

FIGURE 2 is a side view, partially in section, of the form of improvedprimary of this invention shown in FIGURE 1.

Reference will now be made to the drawing, wherein like numerals areused to indicate like parts throughout both figures thereof. As shown inthe drawing, the invention comprises a current transformer 10 having anovel ventilated primary 12 connected thereto. Terminal pads 14 and 16are shown, firmly secured to each end of the primary 12. It is preferredthat the pads 14 and 16 be secured to the primary 12 by welding, asindicated at 18 of FIGURE 2.

Referring now particularly to FIGURE 1, for a more detailed descriptionof a preferred embodiment of this invention, there is shown a currenttransformer 10 which may be of a window or bar type, as shown in FIGURE1, in the preferred embodiment, a bar type transformer 10. The currenttransformer 10 comprises a secondary winding (not shown) which ispreferably of toroidal shape, as disclosed in the aforesaid Patent No.2,436,636. The secondary windings (not shown) are enclosed in anelectrically insulated casing 20, which may be, for example, butylrubber. The secondary winding has its terminals brought out of thecasing 20 as indicated at the secondary terminals 22. A window opening24 is provided in the casing 20, and an insulated bushing 26, preferablyintegrally molded therewith, extends through said window opening 24 onopposite sides of said casing 20. As is well understood, the bushing 26provides the desired creep insulation between the primary 12 and thesecondary terminals 22, to prevent a voltage breakdown between theprimary and the secondary terminals.

Where desired, mounting means may be provided, for example, in the formof brackets 28, which are secured to the casing 20 by means of bolts 30.The bushing 26 will also insulate the primary 12 from the brackets 23,which are usually at ground potential, thereby preventing any voltagebreakdown between such brackets and the primary 12. An opening 31 isprovided in the bushing 26, and the primary 12 is inserted therein. Itwill be understood that the primary 12 may be securely attached to thebushing 26 so as to be non-removable or it may be slidably insertedtherein so as to be removable therefrom. This of course will depend uponthe type and rating of the current transformer, as well as the use towhich it is to be put. As will be understood by those skilled in theart, the bus bars or conductors (not shown) of the electrical systemwith which the current transformer 12 is to be utilized will be attachedto the terminal pads 14 and 16 by means of the opening 32, provided ineach of the terminal pads. The current flowing through the primary 12 isinduced into the secondary windings (not shown) and is taken from thesecondary terminals 22 to the desired meters or relays, or otherindustrial purposes for which it is to be used.

Referring now to FIGURE 2 of the drawing, the ventilated primary bar ofthis invention is shown in the form of a hollow copper tube 12, beingcircular in cross-section, and having the terminal pads 14 and 1.6welded across each end thereof. As is well understood, as the currentflows through the primary bar 12 the temperature of such primary barwill rise above that of the surrounding air. Clearly, those portions ofthe bar, which are within the insulated bushing 26, will heat up fasterand rise to a much higher temperature than the ends of the bar 12, whichextend beyond the bushing 26. In general, it is the heat of this primarybar due to the currents therethrough which determines the rating of thecurrent transformer. However, by means of the hollow primary tube 12 ofthis invention, as the air in the center of the bar is heated, the airwill expand and flow out to the open ends of the primary bar, and willbe replaced by the cooler outside air. Thus, the primary bar of thisinvention will remain at a lower temperature than the solid copper rodsof the prior art. Further, it will be apparent that very little weldingis required to secure the terminal pads 14 and 16 to the primary bar 12.As can be seen, especially in FIGURE 1, it is only necessary to weld theterminal pad to the thin Wall of the primary tube 12, diametricallyacross its open end.

As will be understood, when solid primary bars are used, the centerportion of such bar does not carry an appreciable amount of current.Such center portion is generally filled with eddy currents and the likewhich do not aid the current capacity of such primary, but do contributeto the heat generated therein. By means of the hollow tube primary barof this invention the center core is eliminated, without sacrificing thecurrent carrying ability of the primary. Of course, the eddy currentlosses and heat are also eliminated. Thus it can be seen that the hollowtube primary bar of this invention provides substantially equivalentcurrent carrying ability with a substantial savings in material over thesolid primary bars of the prior art.

In the preferred embodiment, the terminal pads 14 and 16 are providedwith slots 34 and 36 which are designed to exactly fit about the thinwall 38 of primary tube 12. The terminal pads 14 and 16 may then beinserted into the primary tube 12, such that, the slots 34 and 36 willreceive the portions of the thin wall 38 of the primary tube 12. Each ofthe pads may be Welded to the primary 12 about the slots 34 and 36.Thus, it is seen that much less weld material is required. Due to theopen tube construction of the primary 12, where desired, cooling air canbe provided to cool the area around the weld during the welding process,such that the temperature 4.- may be maintained below that which woulddamage the insulation of the bushing 26. It will be understood that theslots could be provided in the sides of tube 12 rather than in theterminal pads 14 and 16, if desired.

Of course it will be obvious to those skilled in the art, that theprimary ventilated bar of this invention may be used with currenttransformers where the insulated bushing 26 is not utilized. In suchinstances, the ventilated primary bar of this invention may be moldeddirectly with the molded butyl casing 20, if desired, or may be laterinserted therein; such as, for example, in the manner set forth inpatent application Serial No. 129,094, filed August 3, 1961, and nowabandoned, in the names of Peter E. Kraus and Harry A. Thorpe, entitledVariable Primary Bar for Current Transformers and the Like HavingLocking Means and assigned to the same assignee as this invention.

While there has been shown and described a preferred embodiment of thisinvention, it will, of course, be obvious to those skilled in the artthat many changes may be made without departing from the spirit andscope of the invention set forth herein. It is therefore, to beunderstood, that such changes may be made in the preferred embodimentherein. This invention is not to be construed as limited by the specificpreferred embodiment set forth herein, but only by the scope of theclaims appended hereto.

What is claimed as new and which it is desired to secure by LettersPatent of the United States is:

1. A ventilated primary bar current transformer having an insulatedcasing and a primary extending through said insulated casing, theimprovement which comprises said primary being in the form of a hollowcylindrical tube having thin walls open to the atmosphere at theopposite ends thereof, said primary being provided with terminal pads atsaid opposite ends, said terminal pads being secured to said thin Wallsof said cylindrical tube diametrically across the openings thereof andwithout closing said openings, whereby cooling air may flow through saidhollow cylindrical tube forming said primary.

2. A current transformer as claimed in claim 1 in which said terminalpads are provided with slotted means which fit around the thin walls ofsaid cylindrical tube.

References Cited in the file of this patent UNITED STATES PATENTS687,141 Everest Nov. 19, 1901 1,526,023 Steinberger Feb. 10, 19251,766,048 Trogner June 24, 1930 1,820,018 Gebherd Aug. 25, 19312,436,636 DEntremont Feb. 24, 1948 2,568,003 Hartmann Sept. 18, 19512,618,689 Cook Nov. 18, 1952 FOREIGN PATENTS 580,973 Canada Aug. 11,1959 1,093,003 Germany Nov. 17, 1962

1. A VENTILATED PRIMARY BAR CURRENT TRANSFORMER HAVING AN INSULATEDCASING AND A PRIMARY EXTENDING THROUGH SAID INSULATED CASING, THEIMPROVEMENT WHICH COMPRISES SAID PRIMARY BEING IN THE FORM OF A HOLLOWCYLINDRICAL TUBE HAVING THIN WALLS OPEN TO THE ATMOSPHERE AT THEOPPOSITE ENDS THEREOF, SAID PRIMARY BEING PROVIDED WITH TERMINAL PADS ATSAID OPPOSITE ENDS, SAID TERMINAL PADS BEING SECURED TO SAID THIN WALLSOF SAID CYLINDRICAL TUBE DIAMETRICALLY ACROSS THE OPENINGS THEREOF ANDWITHOUT CLOSING SAID OPENINGS, WHEREBY COOLING AIR MAY FLOW THROUGH SAIDHOLLOW CYLINDRICAL TUBE FORMING SAID PRIMARY.